Mechanism of Initiation in the Phillips Ethylene Polymerization

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Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Ethylene Activation by Cr(II) and the Structure of the Resulting Active Site Carole E Brown, Adrian Lita, Yuchuan Tao, Nathan Peek, Mark Crosswhite, Melissa L. Mileham, Jurek Krzystek, Randall Achey, Riqiang Fu, Jasleen K Bindra, Matthew Polinski, Youhong Wang, Lambertus van de Burgt, David Jeffcoat, Salvatore Profeta, Albert Edward Stiegman, and Susannah L Scott ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02677 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017

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Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Ethylene Activation by Cr(II) and the Structure of the Resulting Active Site

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Carole Brown, Adrian Lita, Yuchuan Tao, Nathan Peek, Mark Crosswhite, Melissa Mileham,
J. ‡ ‡ † † § Krzystek, Randall Achey,
Riqiang Fu, Jasleen K. Bindra, Matthew Polinski, Youhong Wang, † † † ,† Lambertus J. van de Burgt, David Jeffcoat, Salvatore Profeta, Jr., A. E. Stiegman,* Susannah L. ,§,¶ Scott*

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Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306 United States

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Orbital ATK, Flgiht Systems Group, Corinne, UT 84307 National High Magnetic Field Laboratory, Tallahassee, FL 32310 United States

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Savannah River National Laboratory, Savannah River Site, Aiken, SC 29808 United States

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Department of Chemistry & Biochemistry, University of California, Santa Barbara, CA 93106 United States

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Department of Chemical Engineering, University of California, Santa Barbara, CA 93106 United States Supporting Information Available * corresponding authors [email protected], [email protected]

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ABSTRACT: The structure and mechanism of the formation of the sites which initiate ethylene polymerization in the atomically-dispersed Phillips catalyst (Cr/SiO2) is one of the great unsolved mysteries of heterogeneous catalysis. After CO or C2H4 reduction of silica-supported CrVI ions to CrII ions in the pre-catalyst, exposure to ethylene results in the formation of organoCrIII sites that are capable of initiating polymerization without recourse to an external alkylating co-catalyst. In this work, a Phillips catalyst prepared, via sol-gel chemistry, as a mesoporous, optically transparent monolith was reduced with CO to the spectroscopically-determined CrII endpoint. Ethylene causes rapid reoxidation of these CrII sites to CrIII, even at low temperatures. Solid-state 13C CP-MAS NMR, IR and Raman spectroscopies reveal that the resulting sites contain a vinyl ligand, described as (≡SiO)2CrIII-CH=CH2 although likely with a higher coordination number, which is capable of initiating polymerization. The formation of these vinyl sites is an incommensurate redox reaction involving one-electron oxidation of CrII via ethylene disproportionation. The accompanying formation of organic radical intermediates and their characteristic reaction products suggests that the key step is homolysis of a Cr-ethyl bond. Plausible pathways for the initiation mechanism are suggested.

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Keywords: Phillips catalyst, active site, bond homolysis, ethylene polymerization, sol-gel, initiation mechanism

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INTRODUCTION The Phillips olefin polymerization catalyst was discovered serendipitously in 1951 by Hogan and Banks, who were granted their first patent for olefin polymerization catalysts based on chromium compounds interacting with inorganic oxide supports in 1958.1 Commercial production began in 1956; sixty years later, derivatives of their catalyst are still responsible for as much as 50% of the world’s annual production of high-density polyethylene (HDPE).2 The supported metal oxide is created by dispersing Cr ions onto silica. In industrial practice, Phillips active sites are usually formed in situ during a lengthy induction period that lasts about an hour, during which CrVI is first reduced by ethylene and/or by nonolefinic hydrocarbon solvents/diluents3 to CrII and then alkylated by ethylene at the slightly elevated reaction temperature (typically, 80–120 °C).4-6 The catalyst can also be pre-reduced by CO, eliminating much of the observed induction period but otherwise showing a very similar activation profile.5 The subsequent, slow increase in activity is believed to correspond to alkylation of CrII sites by ethylene. The alternative CO pre-activation procedure is practiced in some commercial HDPE and linear low-density

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polyethylene (LLDPE) operations, and is often used in both academic and industry studies of the precatalyst because it allows researchers to separate cleanly the reduction steps from the ensuing polymerization. According to McDaniel, the CO-reduced catalyst shows similar high activity, polymerizes ethylene under the same conditions, and produces polymers with characteristics close to those made with the ethylene-reduced catalyst.5

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Due to its enormous practical importance, the Phillips catalyst has been studied extensively, and many operating parameters that affect its activity and the properties of the polyethylene it produces are welldocumented.5-7 The mechanism of polymer chain growth by the Phillips catalyst is Cossee–Arlman-type coordination polymerization, in which ethylene inserts repeatedly into the Cr-C bond of alkylchromium active sites.8 Polymer chain termination and the regeneration of the initiating sites occur via chain transfer by a combination of β-H elimination and (at higher operating pressures) β-H transfer to monomer. The net effects of both chain transfer steps are the release of a polymer chain that is vinylterminated at just one end (being methyl-terminated at the other), and initiation of a new polymer chain at a hydridoCr or ethylCr site. The relevance of these mechanistic steps to the Phillips catalyst is supported by extensive kinetic and mechanistic studies, as well as detailed polymer analysis.9-11 However, since the catalyst operates in the absence of alkylating co-catalysts (in contrast to Ziegler-Natta and single-site catalyst systems), the mechanism by which the first polymer chain forms has yet to be convincingly explained. Many initiation mechanisms have been postulated, and the majority invoke a commensurate redox process—typically, oxidation of the CrII pre-catalyst to CrIV,5 or oxidation of a pair of CrII sites to a pair of CrIII sites—by a two-electron reaction with ethylene. An interesting mechanism involving organic radicals was postulated, without experimental support, to explain the net one-electron redox change of an isolated CrII site to CrIII.10 A non-redox initiation mechanism involving ethylene deprotonation by minor CrIII impurities in the pre-catalyst was also suggested,12 although the evidence presented for its feasibility as an initiation step has been challenged,13 and it violates the requirement that re-initiation not be involved in the formation of subsequent polymer chains.5 As McDaniel notes,2 a slow initiation process is compatible with the observed gradual rise in activity, but is inconsistent with the overall fast polymerization rate once the active sites are well-established. Therefore, proposed mechanisms that require re-initiation for each polymer chain are not kinetically viable.

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In our recent report on redox processes that precede initiation by the Phillips catalyst,14 we elucidated the pathway by which the chromate silyl ester is reduced by CO, via a polymerization-inactive CrIV intermediate, to a CrII-containing pre-catalyst.14 The observation of a final CrII state agrees with a large number of previous studies on pre-catalyst reduction by either CO or C2H4.5 We also reported that the CrII site undergoes a redox reaction with ethylene to form organoCrIII species capable of initiating polymerization.14 In this contribution, we describe the first detailed structural characterization of these initiating sites, and present several key elements of a mechanism by which a net one-electron oxidation of the CrII-containing pre-catalyst can be achieved through reaction with ethylene.

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RESULTS AND DISCUSSION As in our previous work on the mechanism of CO reduction of the Phillips catalyst, this study exploits optically transparent xerogel monoliths containing dilute, highly dispersed CrVI ions that we synthesized using a multicomponent sol-gel protocol.15-16 The utility of these materials in our investigation of the Phillips catalyst initiation mechanism builds on our experience with these monoliths in spectroscopic studies of several types of silica-supported catalysts.17-21 While mesoporous (having an average pore size and pore volume of 2.9 nm and 0.26 cm3/g, respectively),14 the materials have a broad pore size distribution with significant microporosity (0.024 cm3/g, Figure S1). The performance of Phillips catalysts made with porous silica supports, including xerogels, has been extensively investigated by McDaniel, who observed a sharp decrease in catalytic activity and polymer melt index for active sites located in pores smaller than 2 nm.22 In this work, we started with a CrVI/SiO2 xerogel that had been subjected to CO reduction to obtain CrII/SiO2. Exposure to ethylene (1 atm) in a closed vessel at 80 °C resulted in immediate and sustained uptake of the gas, as measured by the loss of pressure over time (Figure S2). Consistent with McDaniel’s observations,22 the rate of uptake is relatively slow and ultimately selflimiting, since the pores close as the matrix fills with polymer. Thus, the Cr/SiO2 xerogel monolith serves as a self-sealing reaction vessel that effectively captures many of the early-stage products of polymerization. In addition, slow mass transport of ethylene to the active sites facilitates the

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characterization of reactive intermediates that form during the initiation process. For these reasons, it represents a unique opportunity to study the nature of the first-formed organoCrIII sites and the mechanism by which they are created.

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Generation and Characterization of the Initiating Sites. In a first step toward elucidating their structure, it is useful to explore the conditions under which ethylene can oxidize silica-supported CrII to CrIII. Using high field-high frequency (HFHF) EPR, we previously established that reduction of CrVI to CrII by CO at 350 °C yields CrII as the primary product.14 Two resonances in the Kramers doublet region of the spectrum (g ~ 2) associated with odd-electron oxidation states such as CrV and CrIII were also observed, although these species appear to be minor by-products. Nevertheless, it is important to assess their participation, if any, in the activation of the ethylene.

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Since isolated Cr species associated with odd-electron counts are easily detected by X-band EPR, this technique is a valuable approach for characterizing some aspects of the supported metal ions, and for identifying secondary products of the Cr reduction–reoxidation processes. The X-band EPR spectrum for the catalyst precursor in its nominally fully oxidized CrVI state is shown in Figure S3. The dominant feature is a sharp resonance at g = 1.975, which we assign on the basis of its narrow linewidth and g value to a CrV site.23 It is a known impurity in sol-gel-derived Cr/SiO2 materials, and has been observed in other silica-supported Cr catalysts.23-25 A small, overlapping resonance at g = 1.986 is consistent with the presence of traces of CrIII, which have also been amply documented for the commercial Phillips catalyst itelf.25-26 After CO-reduction of the catalyst, the primary oxidation state is CrII. As a non-Kramers species, it is not visible by EPR at X-band frequencies. Two minor signals for other paramagnetic species are still evident in the spectrum, although their relative intensities have changed (Figure S4a). The first is the ubiquitous CrV impurity, observed in this spectrum at g = 1.974, while the second peak at g =1.986 is again assigned to a CrIII species.25, 27 Compared to the fully oxidized catalyst, the CrIII resonance is significantly more intense relative to the CrV signal, suggesting that more CrIII sites are formed during the reduction of CrVI by CO.

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Addition of ethylene to CrII/SiO2 at room temperature causes much more dramatic changes in the overall intensity and breadth of the EPR spectrum (Figures 1 and S4b). Although the sharp band associated with the CrV species (shifted slightly to g = 1.973) is still evident, the increase in intensity is clearly associated with growth of a new species. Its position (g = 1.981) and linewidth are characteristic of CrIII. The addition of more ethylene results in a further increase in the CrIII signal intensity, which at this point completely obscures the CrV band (Figure S4c). These observations are fully consistent with oxidation of CrII to CrIII by ethylene, as reported previously in our HFHF EPR study.14 We infer that this process and the emerging CrIII resonance are associated with the formation of active sites (see below). The lineshape can be simulated quite well (Figure S4d) using parameters for high spin CrIII (S = 3/2, I = 3/2) by assuming axial symmetry, with gแ =1.9677 and g = 1.9769. Since the resonance of the CrV impurity does not grow in and is eventually obscured by the CrIII resonance at lower field, the experiment clearly suggests that the CrV site is a spectator which plays no role in either the reduction or ethylene activation processes. Thus a recent suggestion28 that it may play a non-passive role in the Phillips initiation mechanism appears to be unfounded. Similarly, a suggestion made by the same authors that silica-based radical defect sites may play a role in initiation is highly unlikely, since such species yield broad, complex X-band EPR signals that would be readily detected29 (they are not seen by us, nor have they been reported in any other study of the Phillips catalyst). While it is unclear from our experiment whether the original CrIII signal that is assigned to a secondary (and very minor) product of CO reduction also reacts with ethylene, comparison of its low signal intensity to that of the CrIII signal arising from reaction of CrII sites with ethylene confirms that it represents a very small fraction of the CrIII present in the activated catalyst.

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Figure 1. X-band EPR spectra of 1 mol% Cr/SiO2 reduced under CO to Cr , recorded after evacuation of CO (black), and after dosing twice with small aliquots of ethylene at room temperature (red and blue).

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We used the dramatic intensification of the CrIII signal in the EPR spectrum to assess the minimum temperature at which ethylene oxidation of the CrII pre-catalyst occurs. A catalyst sample was reduced with CO to CrII, evacuated, and cooled to 77 K. A small aliquot of ethylene was then added to the tube, and the first EPR spectrum in Figure 2 was recorded. At this temperature, ethylene is solid, and the EPR spectrum shows the same two minor resonances associated with CrV and CrIII side-products that are present after reduction but before contact with ethylene (Figure 1). When the temperature was raised to 110 K, just below the melting point of the ethylene (103.6 K), the appearance of the spectral features was unchanged, but their overall intensity had decreased, as expected due to the dependence of EPR intensities on the Boltzmann distribution of ground and excited spin state populations. When the temperature was increased further to 170 K (approximately the vaporization temperature of ethylene, 169.3 K), the EPR spectrum changed abruptly. Notably, the CrIII resonance became more intense, indicating the onset of oxidation of CrII by ethylene. This change is seen more readily in the integral form of the spectrum (Figure 2b). As the temperature was increased further, the signal evolved towards the same band shape and g-value as the CrIII sites generated at room temperature (Figure 1).

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The presence of active sites was verified by polymerizing ethylene under cryogenic conditions. In this experiment, a sample of 1 mol% CrVI/SiO2 was reduced with CO to CrII, then evacuated and cooled in an ethanol/liquid N2 slurry at -85 °C. The cell was filled with ethylene (ca. 1 atm), then resealed. Ethylene consumption was observed to commence immediately via the decrease in pressure in the closed vessel, and production of polyethylene was verified by Raman spectroscopy (Figure S5). Clearly, at temperatures where ethylene is a gas, it reacts with CrII/SiO2, converts at least some of the CrII sites to CrIII sites, and generates a significant number of polymerization-ready sites.

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Figure 2. X-band EPR spectra, plotted in (a) derivative, and (b) integral forms, for 1 mol% Cr /SiO2 dosed with a small aliquot of ethylene at 77 K, and during subsequent, step-wise warming to room temperature.

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To prepare and isolate the activated CrIII sites for characterization, a CrVI/SiO2 monolith was first treated at 350 °C under flowing CO while monitoring the progress of the reaction by UV-vis spectroscopy. The reduction to CrII was judged to be complete when no further spectral changes were observed. The final spectrum shows the previously-assigned CrII ligand field transitions, at 770 and 1229 nm.14 The monolith was then flushed with Ar to remove all CO, both infused and coordinated. After the reaction temperature was adjusted to 80 °C, the pre-catalyst was “titrated” with highly dilute, sub-stoichiometric aliquots of ethylene gas. After each injection, a UV-vis spectrum was collected (Figure 3). The endpoint was deemed to have been reached when the spectrum ceased to change. The final spectrum was discussed in detail in our previous report, where we noted that the spectral bands associated with the ligand field transitions of CrII are no longer visible. Instead, the spectrum is dominated by two overlapping peaks at 676 and 463 nm, previously assigned to ligand field transitions of CrIII sites in a nominally octahedral coordination environment.14

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Figure 3. Changes in the UV-vis spectrum recorded during titration of a Cr /SiO2 xerogel (0.5 mol% VI Cr, prepared by CO reduction of Cr /SiO2 at 350 °C) with aliquots of ethylene at 80 °C.

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At the UV-vis spectroscopic endpoint of the titration, when further addition of ethylene resulted in no more spectral changes, the C/Cr ratio is 1.9 ± 0.2 (measured previously by elemental analysis).14 Coupled with the disappearance of the CrII spectral features, this finding suggests that ethylene reacts stoichiometrically with the CrII sites, resulting in the attachment of a single, ethylene-derived C2 ligand at the majority of these sites. Thus, the transformation of the CrII sites to organoCrIII sites appears to be essentially quantitative, i.e., a large proportion of the CrII sites in this model xerogel catalyst react readily with ethylene. However, it is not necessarily the case that all such sites are active for ethylene polymerization. Indeed, the consensus among Phillips researchers is that only a fraction of the Cr sites participates in polymerization at any given time (although most may be active over the lifetime of the catalyst). The active site fraction varies widely depending on the activation procedure (activation in an extremely dry fluidized bed being most effective),30-31 and on how the value is determined, although McDaniel suggests that catalysts containing 1 wt % Cr typically contain 10–25 % active sites.5 Since polymerization in the xerogel begins even before ethylene activation is complete (according to the Raman results described below), the C/Cr ratio at the UV-vis titration endpoint is necessarily an average value. Fortunately, while CrII sites located in micropores may be sufficiently accessible to undergo the initial redox reaction with ethylene, the microporous regions of the model catalyst are likely to impose severe limitations on the subsequent polymerization rate.

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Structural Characterization of the Initiating Sites

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Solid-state NMR Observations. A monolith containing 1.0 mol% CrII was titrated to its spectroscopicallydetermined endpoint (i.e., until no further spectroscopic changes occurred) using 13C-labeled ethylene, then characterized by 13C solid-state NMR spectroscopy (see Supporting Information for experimental details). The sample was crushed and the powder transferred under rigorously anaerobic/anhydrous conditions to a solid-state NMR rotor. The 13C CP-MAS NMR spectrum contains two distinct resonances of approximately equal intensity, with maxima at 75 and 146 ppm (Figure 4a). Both resonances are extremely broad, characteristic of ligands bonded to a paramagnetic metal ion such as CrIII. The presence of CrIII is also likely responsible for the pronounced asymmetry in the isotropic peaks, since it provides a magnetically heterogeneous chemical environment.

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Figure 4. C CP-MAS solid-state NMR spectra of the organoCr sites generated by titration of COII reduced Cr /SiO2 (1.0 mol% Cr) with ethylene at 80 °C, recorded (a) without dipolar dephasing, as well as with dipolar dephasing of (b) 20 µs, or (c) 30 µs. Spinning rate = 25 kHz.

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The presence of two carbon signals with similar intensities but very different chemical shifts rules out a symmetrically-coordinated ethylene ligand.7 However, the spectrum is consistent with a vinyl ligand. For comparison, a σ-vinyl ligand coordinated to diamagnetic FeII shows terminal and metal-bound carbon signals at 123.5 and 172.5 ppm, respectively.32 In Figure 4a, the resonance at 146 ppm is slightly broader and less symmetrical than the resonance at 75 ppm, consistent with assignment of the former to the carbon directly bonded to the paramagnetic ion. While chemical shifts of carbon atoms near paramagnetic metal ions can vary greatly, extremely large shifts are not expected for organic ligands bound to CrIII because the low-symmetry, orbitally non-degenerate ground state leads to a small pseudocontact component for the shifts. Based on a comparison of the observed 13C resonances in Figure 4a to those of [Fe-CH=CH2],32 we estimate the magnitudes of the dipolar contact shifts to be -48 and -26 ppm for the methine and methylene carbons, respectively. These values are similar to those generated by unpaired spin densities on paramagnetic ions such as NiII and CoII with orbitally non-degenerate ground states.33

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The assignment of the two 13C signals to a vinyl structure is further supported by a dipolar dephasing study. Both signals in the 13C CP-MAS NMR spectrum show strong depolarization that is a function of dipolar dephasing time (Figure 4b,c), indicating that both carbons are directly bonded to protons, and that both are part of a rigid group (such as a vinyl ligand) lacking free rotation about the C=C bond. If the two carbons were instead connected via a single bond (as in an ethyl or ethylidene ligand with a freely rotating CH3 group), averaging of the 13C-1H dipolar coupling would considerably reduce the dephasing effect. Consistent with our assignments, dephasing of the 75 ppm resonance (=CH2) is stronger than that of the 146 ppm resonance (–CH=) because of the stronger homonuclear dipolar coupling between 13C and the two methylene protons that, in turn, influence 13C-1H dephasing. Complete dephasing of the –CH= signal is expected at longer dephasing times (e.g., 60 µs). However, given the high MAS rate (25 kHz), observation of some signal intensity for the –CH= is reasonable even with 30-µs dephasing.34

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IR Spectroscopic Characterization of Alkenyl Ligands. While the NMR data presented above strongly implicate the presence of a σ-vinyl group bound to CrIII, we resorted to vibrational spectroscopy combined with rigorous normal mode analysis and isotopic labeling for unambiguous characterization. In particular, the transparent sol-gel monoliths provide an excellent matrix for the rapid collection of high-resolution IR and Raman spectra. Their interpretation provides strong support for the initial

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formation of [CrCH=CH2] sites and for the subsequent generation of polyethylene chains, at least at some of these sites.

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For a σ-bound vinyl group (-CH=CH2), three characteristic C-H vibrations are typically observed in the IR. These are the normal modes associated with the symmetric (s) and antisymmetric (as) stretches of the terminal methylene (=CH2) group and with the C-H stretch associated primarily with the methine group, –CH=. These bands typically fall between 2900 and 3100 cm-1. In small organic molecules such as propylene and 1-butylene, the methylene νas and νs modes occur at ca. 3090 and 2990 cm-1, respectively, while the methine C-H stretching frequency is intermediate at ca. 3020 cm-1.35-37 The positions shift to lower frequencies for σ-vinyl groups associated with heavier atoms such as Si and Ge, whose spectra have been analyzed in detail.38-41 For the specific example of HCl2Si-CH=CH2, the methylene νas and νs modes are observed at 3071 and 2936 cm-1, respectively, while the C-H stretch associated with the carbon bonded to silicon, Si-CH=, occurs at 2992 cm-1 (Table 1).38, 40 Intense bands at 1595 and 1267 cm-1 are the C=C stretch and the in-plane bending mode of the CH2 group , respectively. Finally, the much weaker CH2 wag, ρw(C-H=CH2), appears at still lower energy, 992 cm-1. Table 1. Comparison of observed and calculated vibrational frequencies (cm-1), and their assignments, for various vinyl groups Modea HCl2SiCH=CH238, 40 Cr/silica xerogel catalyst C2H4 C2D4 observed calculatedd observed calculatede b 3071 3085 3069 n.o. 2273 νas(CH2) b c c 2992 3000 /2993 3030 2235 2229 ν(C-H) 2936 2973 2984 n.o. 2156 νs(CH2) 1595 1586c 1536 1463c 1452 ν(C=C) 1267 1325c 1155 n.o. 1019-988f δ(CH2) 992 n.o. 1035 749c 803-888f ρw(C-H=CH2) a as = antisymmetric, s = symmetric, δ = bend, ρw = wag, n.o. = not observed. b IR. c Raman. d By DFT. Scaling factor 0.965 (rms error = 30). e By DFT. Scaling factor 0.954 (rms error = 33). f These methylene δ and ρw modes are significantly mixed with vibrations of the chromasiloxane framework in the model structure; thus the C-H atomic motions contribute to several, nearly degenerate low frequency modes.

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Figure 5 shows IR spectra in the C-H stretching region of a reduced Cr/SiO2 xerogel recorded during its titration with ethylene (see Supporting Information for experimental details). The highest frequency well-resolved peak observed in the C–H stretching region occurs at 3000 cm-1, and it appeared immediately upon introduction of the first aliquot of ethylene. In addition, two weaker peaks are observed at 3085 and 2973 cm-1; they become better resolved after addition of further aliquots of ethylene. These three bands are consistent with and assignable to the three C–H stretches of a σ-bonded vinyl group. The agreement between the observed frequencies for the catalyst and the frequencies reported for dichlorovinylsilane is generally good. Nevertheless, any small molecule analog will have limitations as a model, particularly in modes that are strongly coupled to, in this case, the HCl2Si- group since it differs significantly from the CrOx/SiO2 surface.

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Figure 5. IR spectra of 0.5 mol% CrII/SiO2 before (0) and after sequential additions of 1− −6 aliquots of ethylene (0.25 mL in flowing Ar) at 50 °C, showing bands associated with a terminal vinyl group (black), and with products of subsequent ethylene insertion (green). The blue line indicates an unassigned mode. The cell was flushed with Ar after each addition to remove gas phase and weaklyadsorbed ethylene.

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Starting with the addition of the third ethylene aliquot, new bands in the C–H stretching region due to inserted ethylene begin to appear, initially at 2928 and 2856 cm-1 and then at 3055 and 2888 cm-1. In principle, each ethylene insertion should generate four new CH2 stretching modes. However, while it is convenient to segregate IR bands into aliphatic and vinyl C-H stretches, the normal modes for the C-H stretching of an alkenyl -(CH2CH2)nCH=CH2 moiety are not decoupled, and their mixing alters both the aliphatic and vinyl normal coordinates to varying degrees. As the chain length, and hence the density of vibrational states, increase, we expect the IR spectrum to become dominated by two intense, broad bands at 2912 (νas) and 2857 cm-1 (νs), characteristic of the methylene stretching modes of bulk polyethylene.42-43 In the spectrum recorded after addition of seven aliquots of ethylene, the bands in the aliphatic region approach but do not quite attain these values.42-43

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Finally, we also observe an IR band at ca. 3115 cm-1, whose intensity grows steadily with addition of ethylene. The origin of this band remains unclear, although it has been reported by others.44 It is not observed in the Raman spectra (see below). While the band may simply be too weak to observe by Raman, it may also be associated with volatile hydrocarbons that are produced during the activation process and incompletely purged from the IR reactor by the Ar flow. A possible assignment is a small olefin such as 1-butene formed during activation (see below), which could become trapped in the sol-gel

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micropores. In studies of 1-butene adsorbed in zeolite micropores, the highest energy C-H stretching occurs at 3100 cm-1, just below the observed value.45 An alternative is a stable π-ethylene complex of CrIII such as in [CrIII(σ-CH=CH2)(π-H2C=CH2)]. While this assignment is appealing, it is unlikely because efficient insertion in the Phillips catalyst would make the steady-state concentration of such a πethylene complex too low to be detectable. Finally, a frequency of 3115 cm-1 may be associated with an aromatic hydrocarbon. On this note, aromatics and cyclopentadienyl ligands have been observed as nonpolymerizable byproducts of the Phillips initiation reaction,46-47 and IR bands in the range 3099 – 3111 cm1 are reported for Cp2Cr/SiO2.48

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Computational analysis of vibrational modes. To aid in vibrational assignments, density functional (DFT) calculations were carried out using models based on previous EXAFS characterization of Phillips CrII sites (see the Supporting Information).49 Because the spectroscopic evidence described above shows that CrII is oxidized to CrIII upon exposure to ethylene, we did not consider [CrII(π-C2H4)] complexes further. To represent the active sites a model containing the [CrIII-CH=CH2] fragment (Figure S6) was used. The calculations predict three C-H stretching modes. After application of an appropriate scaling factor, their predicted frequencies are 3069 cm-1 (νas), 3030 cm-1 (νC-H), and 2984 cm-1 (νs), in good agreement (within 1.6 %) with the experimental values (Table 1). The normal coordinate analysis of the modes associated with the calculated C–H stretching frequencies shown in Figure 6 verifies the correct assignments of these bands.

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Figure 6. Calculated normal modes for the C-H stretches of a silica-supported [Cr CH=CH2] fragment. Color scheme: Cr, green; Si, magenta; O, red; C, gray; H, blue.

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After multiple ethylene doses, the IR spectra reflect contributions from different extents of ethylene insertion, even in the early stages of reaction. As such, some of the observed bands will be the superposition of similar modes originating from different species. After the fourth aliquot, seven bands that are associated with aliphatic and vinyl C–H modes are present in the IR spectrum (Figure 5). We note that while it is tempting to segregate the observed bands into vinyl and aliphatic vibrations, in the early stages of insertion most of the modes likely contain contributions from all of the C–H bonds. Repeated monomer insertion into the Cr-vinyl group will generate an alkenyl fragment which is increasingly distant from the Cr center and which will begin to have vibrations resembling those of a

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purely organic vinyl-terminated chain. This is expected to shift the vinyl vibrational modes to even higher frequencies, which we do not observe in Figure 5. However, the frequency difference is expected to be small and is likely obscured by inhomogeneous broadening. As polymer chain growth continues, bands in the aliphatic region grow in much more rapidly until the spectral bands converge to the well known symmetric and antisymmetric modes for polyethylene.

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Raman spectroscopic analysis. Other fundamental modes characteristic of vinyl ligands occur at IR frequencies that are complicated or obscured by modes associated with the silica lattice. Specifically, these are the C=C stretching and CH2 bending modes, which occur in HCl2SiCH=CH2 at 1595 cm-1 and 1267 cm-1, respectively (Table 1). However, they can be seen in Raman spectra collected after exposing fully reduced CrII/SiO2 xerogels briefly to ethylene followed by rapid evacuation (Figure 7). Exposure times were varied to obtain different levels of ethylene incorporation. In the C-H stretching region, a sharp band appears at 2993 cm-1, close to the position of the band observed at 3000 cm-1 in the IR (Figure 5) and assigned to the C–H stretch of the vinyl group. The CH2 stretching modes (observed to be weaker in the Raman spectrum of vinyltrichlorosilane)38 were not detected. Even at the shortest ethylene exposure time, we also observed weak bands for the characteristic aliphatic C-H stretching (2883 and 2847 cm-1) and bending (1451 cm-1) modes, suggesting that ethylene insertion resulting in limited oligomerization has already occurred.50-51 Unlike the conditions of the IR experiment, in which the bands characteristic of the terminal vinyl group appeared first, the higher ethylene exposure required in the lower sensitivity Raman experiment does not allow the initiating sites to be observed exclusively.

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The first Raman spectrum collected after initial exposure to ethylene also shows two intense, sharp bands at 1586 and 1325 cm-1 (Figure 7a). Their frequencies are consistent with their assignments to C=C stretching and the CH2 in-plane bending. The region below 1300 cm-1, which contains Si-O modes of the silica matrix as well as Cr-O modes, shows a new, sharp band at 980 cm-1 which is insensitive to isotopic labeling of ethylene (see below), as well as spectral changes characteristic of other modes that are perturbed or superimposed on the existing features of Cr/SiO2. They undoubtedly obscure the CH2 wag, which is expected at ca. 992 cm-1.

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Figure 7. Raman spectra of CO-reduced Cr/SiO2 (1 mol% Cr), after exposure to ethylene at room temperature for various times: (a) 5 s, (b) 10 s, and (c) 120 s, followed each time by evacuation. Green lines indicate bands associated with [CrCH=CH2] sites, red lines indicate bands associated with aliphatic oligomers, and the black line is a band associated with a Cr-O mode. Unlabeled bands are modes associated with the silica.

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Assignments for the lower frequency modes are also supported by DFT calculations, using the same [CrIIICH=CH2] model (Figure S6)49 used to predict the energies of the higher-frequency C-H stretching modes (Tables 1). After applying the same scaling factor, the calculated frequency of the C=C stretch, 1536 cm-1, compares reasonably well with the observed value (1586 cm-1). In general, there is less agreement between calculated and experimental values at lower energies: the frequencies of the C=C stretch and the CH2 bend differ from the DFT-predicted values by about 3 % and 13 %, respectively. Scrutiny of the atomic motions that contribute to these normal mode (Figure 6) indicates that they include a considerable amount of atomic motion localized on the chromasiloxane ring, which is a limitation of the small molecule cluster used in the calculation. Finally, the frequency of the DFTpredicted CH2 wagging mode for the vinylchromium site is 961 cm-1, where it is obscured by Raman bands associated with the catalyst support.

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Isotopic labeling of the vinyl group. To provide additional support for the above assignments, the Raman spectrum of the CO-reduced catalyst was recorded after exposure to ethylene-d4 (Figure S7). The DFTpredicted frequency of the vinyl C–D stretch is 2229 cm-1, in very good agreement with the observed band at 2235 cm-1. Similarly, the C=C stretch is predicted to appear at 1452 cm-1, very close to the frequency of the sharp band observed at 1463 cm-1. The CD2 in-plane bending mode is predicted to occur at 985 cm-1, in the region where the silica absorbs strongly, hence it is obscured. We further note that, because of its low frequency upon isotopic substitution, the CD2 bending mode is heavily mixed with framework vibrations of the chromasiloxane model and, in fact, contributes to a range of low frequency vibrations between 1019 and 988 cm-1. Finally, the vinyl CD2 wagging mode is also strongly mixed with framework vibrations of the chromasiloxane model, producing three bands at 888, 812 and 803 cm-1. Consequently, it is likely that the computed frequencies are not highly characteristic of a vinylCr site bound to a rigid silica surface. The two computed bands at 812 and 803 cm-1 have the largest contribution from vinyl wagging, and we assign the sharp Raman band observed at 749 cm-1 to this mode.

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The accuracy of the predicted isotopic shifts for multiple vibrational bands assigned to the normal modes of [CrCH=CH2] upon deuteration supports the normal mode assignments and, therefore, the proposed vinyl structure of the initiating site. Such agreement would be unlikely if the modes were assigned incorrectly. Similar vibrational bands reported in previous vibrational studies of the early stages of ethylene polymerization by the Phillips catalyst44, 52-53 were attributed to π-bonded ethylene associated with fully-reduced CrII sites. This conclusion was predicated, not on rigorous spectroscopic analysis, but on the reasonable expectation that such a species must form initially upon exposure of the reduced catalyst to ethylene. While there is a resemblance between several vibrational bands of –vinyl and – ethylene complexes, the number of observed bands, their relative energies and their predicted isotopic shifts differ sufficiently to distinguish them.54-56 In addition, our EPR results show that contact between ethylene and CrII results in rapid oxidation to CrIII. While we cannot be sure that all of the CrII is oxidized at very low temperatures, the process appears to be essentially quantitative at room temperature, as judged by the complete disappearance of the CrII resonance in the HFHF EPR and the concomitant appearance of CrIII (observed both in HFHF and X-band EPR).14 Thus the necessary π-ethylene complex of CrII is apparently short-lived.

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In light of these reassignments, we note that an early IR study of the Phillips catalyst by Kantcheva et al. also showed clear evidence for the unique formation of [CrCH=CH2] sites, as evidenced by the observation of its characteristic C-H and C=C stretching and bending modes, well before the appearance of bands for the aliphatic C-H stretching of polyethylene.44 However, the authors interpreted their spectrum in terms of an ethylidene site, [Cr=CHCH3]. Others have suggested vinyl ligand formation by a non-oxidative process, but with only indirect12 (and incorrect)57 spectroscopic evidence for vinyl generation. A diffuse reflectance IR study of the reaction of ethylene with CrVI/SiO2 at the typical Phillips operating temperature of 100 °C claimed to observe bands characteristic of a [CrIIICH=CH2] site. However, reduction by ethylene resulted in a large number of bands in the C-H stretching region associated with ethylene oligomers, as well as small organic molecules resulting from ethylene oxidation

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by CrVI and other byproducts of the activation process. Nevertheless, the authors speculated that [CrIIICH=CH2] could be identified based solely on the appearance of a band at 2960 cm-1, which they assigned to the ν(C–H) stretch of the σ-vinyl group. Obviously, a band at this frequency is not unique to a σ-vinyl group, and since no such characteristic bands were assigned, the spectrum cannot be considered to support the claim. In contrast, the vibrational spectra reported here (Figures 5 and 7a) were collected at very early stages during the activation process, there is relatively little spectral congestion from interfering species, and the presence of the [CrIIICH=CH2] site can be established unambiguously from the characteristic bands that that are intrinsic to the site.

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Polyethylene characterization. Following initial activation of ethylene on the CrII sites, ethylene insertion and formation of polyethylene occur rapidly. The Raman spectra in Figure 7b,c reveal characteristic aliphatic CH2 stretching modes between 2800 and 2950 cm-1. In the early stages of reaction, these bands are broad; however, with increasing exposure to ethylene, they converge to frequencies close to those characteristic of the symmetric and antisymmetric CH2 stretches (2842 and 2876 cm-1, respectively) of bulk polyethylene.42 At the end of the experiment, three principal bands are present (in order of decreasing relative intensity) at 2847, 2882, and 2918 cm-1, as well as weaker bands at higher and lower frequencies. Figure 8 shows the Raman spectrum of a fully-reduced catalyst monolith from which the CO was evacuated and replaced with ethylene. The spectrum recorded after approx. 5 min reaction is shown in Figure 8a. Ethylene was allowed to continue to polymerize until its uptake ceased, presumably because the available pore volume was effectively filled with polyethylene. The consumption of ethylene as polymerization occurs is seen by the decrease in the intense, Raman-active Ag fundamental at 2993 cm-1 (Figure 8b). We note that the frequency of this band, which is below that of free gas phase ethylene, as well as the observation of the very weak, non-allowed B2u band at 3070 cm-1 indicates that ethylene experiences intermolecular interactions in the pores and weak adsorption on the silica surface.52, 58-59 The vibrational spectrum in the CH2 stretching region for bulk polymer trapped in the microporous matrix consists of two broad bands at 2841 and 2886 cm-1, with a resolved shoulder at 2918 cm-1 (Figure 8b). The broadening of these bands and the shifts in the peak postion reflect the increasing density of states as more the aliphatic C-H stretching modes are added by the chains.

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Figure 8. Raman spectra in the C-H stretching region for (a) Cr /SiO2 (1.0 mol% Cr), fully reduced with CO, evacuated and back-filled with ethylene at 80 °C, recorded after (a) ca. 5 min reaction; (b) after ethylene uptake had ceased; (c) the polyethylene component of this material, after removal of the catalyst by dissolving it in HF, following by annealing the polymer above its melting point (130 °C); and (d) a reference standard of commercially produced bulk high-density polyethylene (the -1 51 bands at ca. 2700 cm are overtone and combination bands intrinsic to the polyethylene standard). The spectra were scaled to have the same overall integrated intensity. * Weakly adsorbed ethylene.

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After completion of the in situ experiment, the polymer was separated from the xerogel catalyst by dissolving the monolith in HF. Differential scanning calorimetry (DSC) of the insoluble fraction showed a sharp melting point at 129.9 °C, characteristic of high-density polyethylene (Figure S8b).60 The Raman spectrum of this polymer, annealed above its melting point, is shown in Figure 8c. The spectrum shows a narrowing of the peaks into the characteristic CH2 stretching modes of polyethylene. (For comparison, a spectrum of commercially produced, high-density polyethylene is shown in Figure 8d). The relative intensities of the bands for the annealed polymer, in which the band at 2881 cm-1 is the most intense, differ slightly from those observed in the monolith. This is not unexpected, since the intensities of the CH2 stretching bands of polyethylene depend on interchain interactions resulting in Fermi resonances, and can vary strongly with the crystallinity of the polymer.61 The polymer dispersed in the xerogel pores likely has low crystallinity, with minimal chain-chain interactions. Removal of the xerogel and annealing of the polymer causes the crystallinity to increase.

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Mechanism of Formation of the Initiating Vinyl Sites

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In the many decades since the discovery of the Phillips catalyst, numerous structures have been proposed for the Phillips initiating site, including Cr hydrides, alkyls, alkylidenes and metallacycles.6, 62-63 In most cases, their formation was suggested to involve two-electron oxidative addition of one or two ethylenes to give organoCrIV sites. Some of the proposed reactions require a source of H+ (other than from C2H4) to create the initiating species,53 although the only available protons in the fully dehydrated and highly dehydroxylated catalyst are associated with silica surface hydroxyls, and their presence is strongly negatively correlated with polymerization activity.64 Cr-vinyl and higher alkenyl sites have been proposed by many researchers to arise from ethylene activation processes, yielding organoCr initiating sites in oxidation states from II to IV.7, 10, 65-68 Deprotonation of ethylene to form a Cr-vinyl site and a bridging surface hydroxyl was first suggested in the 1960s by Kazansky,69-70 followed in the 1970s and 1980s by Yermakov71 and McDaniel.64 The idea has been revived recently.72-74 However, no rigorous spectroscopic evidence for the vinyl initiating site has ever been reported. Thus, the work presented here represents the first and thus-far only direct and unambiguous characterization of a vinylCr species during initiation of polymerization by the Phillips catalyst.

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Formation of a vinylCrIII site by reaction of ethylene with a CrII precursor site requires a net one-electron change in oxidation state at the metal but a net two-electron change for the organic ligand. A possible mechanism proposed by Kissin and Brandolini8 involves initial insertion of CrII into the C-H bond of an ethylene π-complex to form a vinylCrIV hydride (Scheme 1, path 1). A computational assessment of its feasibility suggested that spin-crossing from the ground-state quintet CrII complex to the triplet excited state would precede ethylene oxidative addition.75 Conversion to a vinylCrIII species from the resulting vinyl hydride complex was suggested to proceed via H-atom abstraction by ethylene. However, the free energy barrier for direct oxidative addition of ethylene was computed to be nearly 200 kJ/mol at 373 K, and this pathway was not considered energetically viable. Concerted mechanisms have much more favorable barriers. For example, the cycloaddition of two coordinated ethylenes readily yields a chromacyclopentane intermediate (path 2), despite the need for spin-crossing.73 This reaction has precedent in homogeneous organochromium chemistry,74 and is a key step in catalytic ethylene trimerization.76 A transition state for intramolecular H-atom transfer within the chromacyclopentane ring leading to the proposed (ethyl)(vinyl)CrIV intermediate was sought computationally, but was not found.75 H-atom transfer from coordinated ethylene to the chromacyclopentane ring also has a very high computed free energy barrier (193 kJ/mol). However, the readily-formed chromacyclopentane (whose ring expansion is too slow to account for polymerization) should undergo facile cycloreversion, and IR bands assigned to small chromacycles formed on silica at low temperature disappeared upon evacuation, presumably by reversion to labile ethylene complexes.77

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Scheme 1. Possible mechanisms for the formation of the vinylCrIII initiating site in the Phillips catalyst.

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A possible pathway leading to a vinyl species is concerted H-atom transfer within the triplet CrII(C2H4)2 complex (step 3), with an overall free energy barrier of 151 kJ/mol from the quintet ground state. This value compares reasonably well to the barrier estimated from the observed initiation kinetics (ca. 120 kJ/mol), considering the simplicity of the model.75 The barrier depends on the local structure of the silica that dictates the energy of the chromasiloxane ring, which a small cluster model is unlikely to reproduce with high fidelity.

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Previously, we proposed that a vinylCrIII site could form by Cr-C bond homolysis (step 4).75 Such reactions are well-documented and extremely rapid in at least some organoCrIV complexes.78-83 The higher bond strength of the Cr-vinyl bond relative to the Cr-ethyl bond suggests that an ethyl radical would be extruded preferentially from an (ethyl)(vinyl)CrIV intermediate.84-85 We computed free energies for homolytic bond dissociation in (ethyl)(vinyl)CrIV complexes with a bis(silanolate) supporting ligand at 100 °C.75 The results depend strongly on the Cr coordination number, varying from 98 kJ/mol for fourcoordinate Cr to just 5 kJ/mol for 6-coordinate Cr, where the additional spectator ligands are provided by siloxanes (ethylene appears to be ineffective as a spectator ligand in enhancing the homolysis rate). The free energy barriers likewise depend strongly on the Cr coordination number, from 152 kJ/mol for four-coordinate Cr to just 47 kJ/mol for 6-coordinate Cr with two additional siloxane ligands. If we assume that some Cr sites in the Phillips catalyst can achieve higher coordination numbers through interaction with adjacent siloxanes, then ethyl radical release with siloxane assistance from such (ethyl)(vinyl)CrIV sites would be fast under polymerization conditions. The so-formed vinylCrIII sites, with variable siloxane coordination, would be capable of ethylene polymerization. The extruded ethyl radicals may undergo self-reaction to give (predominantly) n-butane, abstract a vinyl ligand (step 5) to regenerate an inactive CrII site, or combine with a CrII site to form an activated ethylCrIII site (step 6). In principle, ethyl radicals could also abstract H atoms from polyethylene chains once these chains are abundant. Such reactions are the basis for radical polymerization in commercial LDPE processes. However, the ethylene partial pressure in our experiments (and even in typical Phillips polymerization reactors) is too low to sustain such reactions as chain-propagating processes.

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Evidence for the formation of organic radicals. An critical step in the initiation mechanism described above is the extrusion of an organic radical (step 4). Evidence for this reaction during Phillips initiation was sought using EPR spectroscopy. A sample of CrVI/SiO2 (3.0 mol% Cr) in an EPR tube was first reduced to CrII/SiO2 with CO at 350 °C, then evacuated. The tube was filled with 1 atm ethylene at 80 °C for ca. 1 min, then immersed in liquid N2 and the EPR spectrum was recorded again (Figure 9). A new, sharp signal appears superimposed on the broad CrIII resonance, with a g value very close to the free electron value and a very narrow peak-to-peak line width, 5 G. It is diagnostic of the presence of organic radicals; for comparison, even metal-based radicals such as CrV, which have narrow line widths, are much larger (typically, 15 G).23, 86 However, in the absence of resolved hyperfine coupling (possibly due to the presence of and interactions with CrII and/or CrIII), the sharp signal cannot be assigned conclusively to ethyl radicals.87-89 The radical signal was also observed at a lower Cr concentration (0.5 mol%) and after longer reaction times (ca. 5 min). Indeed, radicals appear to be produced throughout much of the early stages of polymerization, undoubtedly facilitating their observation in these experiments. To the best of our knowledge, our work is the first to demonstrate their presence and possible role in the initiation process of the Phillips catalyst.

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Figure 9. X-band EPR spectra, recorded at 9.40 GHz and 77 K, for a Cr /SiO2 (3.0 mol%) xerogel after reaction with ethylene at 80 °C for ca. 1 min, followed by quenching in liquid N2.

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Organic radicals generated during the activation of the Phillips catalyst at low ethylene pressures are expected to undergo characteristic recombination and disproportionation reactions, rather than reactions with ethylene. Ethyl radicals should, therefore, undergo self-reaction to produce n-butane. Evidence was sought for this predicted initiation product by gas chromatography. Indeed, n-butane was the most significant organic product formed during the earliest stages of polymerization at 80 °C, shortly after CrII sites were first converted into initiating sites (Figures 10 and S9). Ethane, an expected product of ethyl radical disproportionation, was not detected; however, gas phase studies of ethyl radical selfreactions report that the expected n-butane/ethane ratio is > 7.90-91 Based on the amount of n-butane detected, the amount of ethane produced is likely below its detection limit. However, a previous study reported both ethane and n-butane as products in the early stages of reaction between ethylene and both fully oxidized and pre-reduced Phillips catalysts.46

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Figure 10. Time-resolved evolution of small organic reactants and products (relative to the amount of II Cr) during the early stages of ethylene polymerization at 80 °C and 1 atm over CO-reduced Cr /SiO2: ethylene (●), n-butane (●) and 1-butene (●), all detected by GC. Lines are shown only to guide the eye.

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Assuming that each CrII site yields one ethyl radical during its activation, the expected value of the nbutane/Cr ratio is 0.5. However, during the first 3 min of reaction, the measured ratio is significantly larger, at ca. 6.4. It suggests repeated formation of vinylCrIII sites in the very early stages of the reaction, prior to the onset of polymerization. Consistent with this hypothesis, we also detected 1-butene, albeit in smaller quantities. This alkene could arise via reaction of an ethyl radical with a vinylCrIII site, causing the initiating site to revert to a precursor CrII site (Scheme 1, step 6). Such a process may occur frequently during initiation, but it is likely a minor contributor to the overall ethylene uptake during

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polymerization. An alternative route to 1-butene involves slow ethylene insertion into a CrIV-ethyl bond, followed by β-H elimination from the resulting CrIV-n-butyl site that is starved of ethylene under the particular reaction conditions of this study. The expected incorporation of 1-butene into growing polymer chains would account for the low yield in the gas phase.

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The reactions in Scheme 1 (or some variant on these) appear to represent the critical initiation processes by which ethylene is activated on isolated CrII sites, leading to vinylCrIII sites that are capable of insertion-based polymerization. Ethylene insertion and chain growth from these initiating sites generate the first polymer chains (Scheme 2). Under industrially-relevant polymerization conditions, the majority of the vinyl-terminated chains that become polymerization sites likely undergo either β-H elimination or chain transfer to monomer. In either case, the first such polymer chains liberated from the initiating sites will be vinyl-terminated at both ends. However, the resulting ethylCrIII sites can then proceed to make polyethylene by a conventional Cossee-Arlman mechanism, leading to chains with mixed methyl/vinyl termination, as observed experimentally.2, 10

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Scheme 2. Proposed mechanism for initiation followed by ethylene polymerization over the Phillips catalyst.

Summary We have presented the first rigorous experimental evidence for the key steps leading to the generation of the active sites of the Phillips ethylene polymerization catalyst through in situ monitoring of these processes as they occur in a porous Cr/SiO2 xerogel monolith. Ethylene activation results in oxidation of CrII sites to CrIII sites that produce the first polymer chains. The vinylCrIII initiating sites were created and isolated in the xerogel matrix, and their structure was elucidated by solid-state NMR and vibrational spectroscopies. The key to the incommensurate redox reaction between CrII and ethylene is Cr-C bond homolysis, which causes CrIV to be reduced to CrIII and generates an organic radical. Based on these findings, a new and comprehensive mechanism for ethylene polymerization over atomically-dispersed, silica-supported chromium ions is proposed that accounts for most of the known reactivity properties of the catalyst (Scheme 2).

Experimental and Computational Methods Preparation of Cr/SiO2 Xerogels. Transparent, porous silica xerogels containing between 0.5 and 3.0 mol% Cr (mol Cr/(mol Cr + mol Si)× 100 %) were made by co-condensing tetramethylorthosilicate (TMOS) with CrO3 in an aqueous 2-propanol solution, following published procedures.15, 17 The sols were allowed to gel, then aged over a period of 2–3 months, after which they were slowly dried and calcined in a furnace at 500 °C. This resulted in transparent yellow-orange monoliths (depending on the Cr concentration) in which the CrVI sites are well-dispersed throughout the silica matrix.

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Reduction of Cr/SiO2 Xerogels and Ethylene Activation. In-situ monitoring of polymerization by the Phillips catalyst was carried out in a closed, high-temperature and high-pressure spectroscopy cell (International Crystal Systems) equipped with sapphire windows, as described previously.14 In a typical experiment, a Cr/SiO2 (0.5 mol% Cr) xerogel monolith (ca. 1.5×0.5×0.5 mm) was mounted in the cell and calcined under flowing O2 at 500 °C for approx. 2 h. To generate the CrII/SiO2 pre-catalyst, reduction of the monolith was carried out under flowing CO at 350 °C while monitoring changes in the UV-vis spectrum. The reduction was considered complete when the spectrum ceased to change. Ethylene activation was achieved by delivering small aliquots of ethylene to the reduced Cr/SiO2 xerogel at 50 °C while monitoring the spectral changes. Specifically, for delivery of ethylene to the Cr-xerogel monolith, 0.5 mL aliquots of ethylene were injected using a gas-tight syringe into a hose carrying an Ar stream (ca. 80 mL/min at ambient pressure, 1 atm) that flowed into the sample chamber (ca. 90 cm3) and was allowed to diffuse throughout the porous sample. UV-vis spectra were recorded after each addition of ethylene. The experiment was terminated when no further spectral changes were observed.

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EPR and NMR Spectroscopic Characterization. EPR spectra were recorded on a Bruker E680X instrument. Solid-state NMR spectra were recorded on a Varian Inova 500 MHz wideband spectrometer. A sample of Cr/SiO2 (1.0 mol% Cr) was reduced to CrII with CO, then titrated with 13C-labeled ethylene to the UV-vis-determined endpoint. The sample was then loaded under strictly anaerobic and anhydrous conditions into a 2.5 mm zirconia rotor and spun at 25 kHz, with 0–30-µs dephasing. Chemical shifts are reported relative to TMS.

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Vibrational Spectroscopic Characterization. IR spectra were acquired for a Cr/SiO2 xerogel pressed into thin disks (9 mm diameter, ca. 1.3 mm pathlength)to facilitate collection of spectra in transmission mode. Cr/SiO2 was calcined (O2, 500 °C) then reduced(CO, 350 °C) to form the CrII-containing precatalyst. IR spectra were recorded in situ in the high temperature-high pressure spectroscopic cell described above. Small aliquots of ethylene were added to the cell and allowed to react with the CrII sites. IR spectra were collected after each addition. Raman spectra were collected for monolithic Cr/SiO2 xerogels (1.0 mol% Cr, ca. 1.5×0.5×0.5 mm) placed in a quartz cell with a cylindrical reaction container for high-temperature processing and a side-arm equipped with a spectroscopic cell. Raman spectra were obtained using a micro-Raman spectrograph, JY Horiba LabRam HR800. Samples were excited by a 17 mW 633 nm HeNe laser.

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Density Functional Theory Calculations. Calculations were carried out at the FSU Research Computing Center (RCC) using Gaussian v09 software.92 The B3LYP density functional and 6-31G* basis set were used in all calculations. The Gaussian output contains the calculated IR and Raman frequencies. The Gaussian checkpoint file, reformatted in the .fchk format, was imported into VIBRATZ 2.0 for Linux (Shape Software) to convert the Cartesian coordinates into internal coordinates for normal mode analysis of each structure.

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Polymer Characterization. Differential scanning calorimetry (DSC) was carried out using a TA Instruments Q2000 calorimeter.

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ASSOCIATED CONTENT

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The materials, methods and experimental details of each step in the reaction sequence, as well as product characterization, are given in the Supporting Information. This material is available free of charge online at http://pubs.acs.org.

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AUTHOR INFORMATION

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corresponding authors

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* [email protected], [email protected]

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ACKNOWLEDGMENTS

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We thank Dr. Eric Dowty of Shapesoft for helpful discussion.This work was carried out with funding provided by the Catalysis Science Initiative of the U.S. Department of Energy, Basic Energy Sciences (DEFG02-03ER15467). This work was also supported by the National High Magnetic Field Laboratory

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